A single crystal of SiC is used as a substrate of an SiC semiconductor useful as a semiconductor device such as a high-temperature working device, a power device and a blue light-emitting device. Various methods of growing such an SiC single crystal have hitherto been known. Among these conventional methods of growing an SiC single crystal is a sublimation recrystallization method. In the sublimation recrystallization method, silicon carbide constituting a source material is sublimated in a graphite crucible, so that a silicon carbide single crystal is recrystallized on a silicon carbide single crystal substrate which is disposed at a low-temperature region in the crucible. In this case, the temperature of the silicon carbide constituting a source material, the temperature of the siliconcarbide substrate and the atmosphere in the graphite crucible are precisely controlled in order to obtain a high-quality siliconcarbide single crystal.
In the above process, the sublimation of the silicon carbide powder constituting a source material takes place from the surface of the silicon carbide powder and the silicon carbide powder at the opening surface (sublimation surface).
Silicon carbide partly decomposes upon sublimation (to thereby form decomposition products such as Si vapor, Si.sub.2 C vapor and SiC.sub.2 vapor). The released vapors (or gases) in total have a greater silicon content than the carbon content. As a result, a carbon layer remains on the silicon carbide powder at the opening surface. In other words, in the above-described method, the sublimated silicon carbide necessarily passes through the carbon layer during sublimation. This makes it difficult to stably supply silicon carbide vapor (source material gas).
To overcome the above problems in the sublimation recrystallization method, various techniques have been proposed. For example, Japanese unexamined patent publication No. 6-128094 (No. 1) discloses a method of producing a silicon carbide single crystal in which a silicon-component gas (e.g., silane or derivatives thereof) and/or a carbon-component gas (e.g., hydrocarbon gases such as methane and ethane) is/are introduced into a growth reaction area when growing a silicon carbide single crystal on a seed crystal by sublimating a silicon carbide source material, in order to cancel fluctuation of the sublimated gas composition caused with the progress of sublimation reaction.
Japanese unexamined patent publication No. 6-298600 (No. 2) discloses a method of growing an SiC single crystal by a sublimation recrystallization process comprising heating and sublimating an SiC powder source material to grow an SiC single crystal on a seed crystal comprising an SiC single crystal maintained at a temperature lower than the temperature of the source material, in which an Si-containing gas is supplied as another SiC source in addition to the SiC powder.
Japanese unexamined patent publication No. 6-1698 (No. 3) discloses a method of producing a bulk single crystal of silicon carbide, comprising growing a silicon carbide single crystal on a seed crystal by a sublimation recrystallization method, in which a silicide of a transition metal (e.g., tungsten silicide and tantalum silicide) is added to the silicon carbide powder.
Japanese unexamined patent publication No. 6-56596 (No. 4) discloses a method of producing a silicon carbide single crystal, comprising growing a silicon carbide single crystal by a sublimation recrystallization method using a seed crystal, in which silicon nitride is added to the silicon carbide constituting a source material.
In addition to the above-described sublimation recrystallization methods, for example, Japanese unexamined patent publication No. 2-293398 (No. 5) discloses a method of producing a high-quality crystalline silicon carbide by introducing a carbon-containing compound vapor and silicon vapor into a plasmareaction space (e.g., arc discharge space).
Furthermore, on page 609 of Technical Digest of "International Conference on SiC and Related Materials" (ICSCRM-95) (No. 6), it has been reported that a bulk single crystal of silicon carbide is obtained by carrying out a reaction represented by the following formula: EQU SiH.sub.4 +C.sub.3 H.sub.8 .fwdarw.SiC
at a temperature of 1800.degree. C. to 2300.degree. C.
However, even the above-described methods of No. 1 to No. 4 cannot perfectly compensate the composition fluctuation of silicon carbide vapor due to partial decomposition of silicon carbide upon sublimation, so that it is difficult to achieve a constant stable state during recrystallization of silicon carbide. Furthermore, in the case of growing a semiconductor-grade silicon carbide single crystal, which contains impurities in very little amounts, it is necessary to supply a source material of a high-purity silicon carbide powder commensurate with the desired single crystal. However, such a highly pure material costs much, which leads to a high cost of production of a silicon carbide single crystal.
The above-described method of No. 5 requires an apparatus of a large-volume plasma to obtain a large-size single crystal of silicon carbide. However, the plasma space (the arc discharge space) is not suitable for the crystal growth in a large size due to the narrow area of the arc discharge space. Therefore, this method is disadvantageous in that the producing apparatus necessarily becomes large-scale and costs much.
The above-described method of No. 6 is not suitable to mass production of bulk single crystals because the method employs expensive silane (SiH.sub.4) gas as a source material. Further, the growth rate of the crystal may decrease because silane contains hydrogen.
As described above, in producing a silicon carbide single crystal, the crystal growth using a recrystallization technique (methods of No. 1 to No. 4) has difficulties of accomplishing a stable (constant) crystal growth and obtaining a high-purity single crystal. This causes deterioration of electrical characteristics of a semiconductor substrate comprising the silicon carbide single crystal obtained by the methods. The other methods (methods of No. 5 and No. 6) also have difficulties of providing a low-cost production of a large-size crystal.
As a result of the intensive studies made by the present inventors to solve the above-described problems, the present invention has been accomplished.